Journal of the Korea Academia-Industrial cooperation Society (한국산학기술학회논문지)

The Korea Academia-Industrial cooperation Society (한국산학기술학회)
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Estimation of Refractive Index in MIR range from the Reflectance Measurements for IR Optics Materials

반사율 측정에 의한 적외선 광학재료의 중적외선 굴절률 추정

  • Received : 2020.03.16
  • Accepted : 2020.06.05
  • Published : 2020.06.30
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Abstract

An optical arrangement has been set inside a photo-spectrometer to measure the reflectance of IR optics materials in mid IR range. The optical arrangement consists of equally spaced 4 gold coated full reflecting mirrors with the incidence angle of 45°. Baseline beam intensity IB has been measured while the beam proceeds through the 4 mirrors. Reflectance of a mirror has been estimated from the IB. And the beam intensity IS with the specimen in the optical path has been measured with the 4th mirror replaced with the specimen. Reflectance of the specimen has been estimated from the value of IS/IB. Then the estimated reflectance has been put in Fresnel equation relating reflectance and refractive index(RI) to estimate the RI of the material. Measurement has been made for sapphire, germanium, magnesium fluoride, and zinc sulfide. The estimated RI of the materials are closely matching with reference data and the maximum difference less than 2% over the wavelength range 3-5㎛ for all materials tested. As an FT-IR photo-spectrometer with a broadband wavelength infrared light source is used, this method has the advantage of measuring the refractive index at multiple wavelengths in a single measurement.

본 연구는 적외선 광학재료의 중적외선 영역의 굴절률을 파악하기 위해 반사율을 측정하고 굴절률과 반사율의 관계로부터 굴절률을 추정하는 방법을 제안한다. 전반사 거울 4개를 일정한 간격으로 배열하고 적외선 광원으로부터 연속 파장의 평행광이 45°로 입사되도록 광 경로를 구성한다. 광 경로를 따라 4개의 전반사 거울에 반사되어 온 광강도 IB를 측정한 후, 광 경로의 마지막에 놓이는 거울을 시편으로 교체하고 광강도 Is를 측정한다. IB와 Is, 전반사 거울의 비를 이용하여 재료의 반사율을 계산한다. 계산된 반사율과 Fresnel의 굴절률과 반사율 관계식에 넣고 시행착오법으로 굴절률을 추정한다. 이 방법을 적용하여 사파이어(Al2O3), 게르마늄(Ge), 불화마그네슘(MgF2), 황화아연(ZnS)재료를 대상으로 실험하고 굴절률을 추정하여 참고문헌자료들과의 비교를 통하여 모든 재료에 대해 파장범위 3 - 5㎛에서 최대 차이 2% 이하로 잘 일치하는 결과를 얻을 수 있었으며, 이를 통해 본 굴절률 측정방법의 타당성을 확인할 수 있다. 본 연구에서 제시된 방법은 연속 파장의 적외선 광원을 사용하기 때문에 한 번의 측정으로 여러 파장에 대한 굴절률을 추정 할 수 있는 장점이 있다.

Keywords

References

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